1. Field of the Invention
The present invention relates to a data recording medium and a method of manufacturing the same and, more particularly, to a data recording medium of a pit formation type having an undercoat layer formed under a recording layer and a method of manufacturing the same.
2. Description of the Related Art
As a conventional data recording medium of a type in which data is recorded by forming pits, a data recording medium having a Bi thin film on a substrate was disclosed in D. Maydan, Bell Syst. Tech. J., 50,1761 (1971); U.S. Pat. No. 3,720,784. In 1977, a data recording medium having a recording layer formed of Te alloy, which has been put into practical use, was disclosed by R. McFarlane et al. in Philips, "Proceedings of SPIE", 123, 104 (1977). Te alone is easily oxidized in a high-temperature atmosphere. Production of TeO.sub.2 degrades the recording sensitivity of a data recording medium because TeO.sub.2 absorbs no light in a near-infrared wavelength. In addition, if peripheral portions of pit formation regions are oxidized upon formation of pits, and TeO.sub.2 is produced in these portions, reflectivities of the peripheral portions of the pit formation regions are decreased. As a result, the pit formation regions become obscure, and undesired effects appear in reproduction of recorded data. For this reason, in order to prevent degradation in recording sensitivity, it is desired that the recording layer of a pit formation type recording medium is composed of a material having oxidation resistance.
Under the circumstances, a data recording medium which the oxidation resistance of a recording layer is improved by using a Te-Se alloy thin film as the recording layer is disclosed in M. Terao et al., J. Appl. Phys., 50,6881 (1979). However, since cracks are easily formed in a Te-Se alloy thin film, Pb is added to form a Pb-Te-Se alloy thin film in practice. In this manner, Te alloys are used as materials for recording layers.
In addition, a method of forming a recording layer, called a Te-C layer, having excellent oxidation resistance by sputtering a Te target in an atmosphere containing a CH.sub.4 gas is disclosed in M. Mashita and N. Yasuda, "Proceedings of SPIE", 329, 190 (1982); U.S. Pat. No. 4,433,340. Furthermore, a method of obtaining a CS.sub.2 -Te thin film having excellent oxidation resistance by depositing Te while performing plasma polymerization using a CS.sub.2 gas is disclosed in Y. Asano et al.,; U.S. Pat. No. 4,373,004; J.J.A.P., 22, 480 (1983); Published Examined Japanese Patent Application No. 58-17038.
Recording medium having these recording layers have poor recording sensitivities because satisfactory reflection of laser beams does not occur in data recording. Published Unexamined Japanese Patent Application No. 59-90246 discloses a data recording medium whose recording sensitivity is improved by forming a fluorocarbon undercoat layer under the recording layer. In addition, Published Unexamined Japanese Patent Application No. 62-154343 discloses a method of further improving recording sensitivity by specifying the ratio of fluorine to carbon in an undercoat layer.
With an increase in data transfer speed and data recording capacity, the rotational speed of a disk is increased. The recording sensitivities of these data recording medium are not good enough for such data recording. A strong demand, therefore, has arisen for a data recording medium having higher recording sensitivity.